1. Field of the Invention
The present invention relates to an image forming apparatus such as printer, copying machine, facsimile or multifunction peripheral having functions of printing, copying, faxing and the like, and more particularly a tandem-type color image forming apparatus.
2. Description of the Related Art
There has been known a tandem-type color image forming device in which four photoreceptor drums for cyan, magenta, yellow and black are aligned along an intermediate transfer belt. Cyan, magenta, yellow and black toner images are first formed on the photoreceptor drums, respectively, and then transferred onto the intermediate transfer belt (which is referred to as primary transfer process) such that they overlap each other to form a color toner image. Next, the color toner image is carried by the intermediate transfer belt to a nip between the belt and a transfer roller pressed against the belt. The color toner image is transferred onto a sheet which passes through the nip (which is referred to as secondary transfer process), so that a color image is formed on the sheet.
The device described above generally has a color mode where the four toners are used and a monochrome mode where one toner (typically, black toner) is used. In the color mode, all the photoreceptor drums are in contact with the intermediate transfer belt (contact state) in the primary transfer process. In the monochrome mode, on the other hand, only one of the photoreceptor drums (typically, for black) is in contact with the intermediate transfer belt while the others are separated from the belt (separate state). Such switching can prevent an unwanted consumption of process members (e.g. photoreceptor drum or developing device). For example, if a photoreceptor drum that does not serve to form a monochrome image keeps in contact with the intermediate transfer belt in the monochrome mode, an abrasion of the surface of the photoreceptor drum might be accelerated. Accordingly, where a print job containing color and monochrome documents is executed, the photoreceptor drums and the intermediate transfer belt are required to be switched between the contact and the separate states, depending on which of the color mode or the monochrome mode has been selected. However, when an image forming operation is started during or immediately after the switching process, a possible vibration of the intermediate transfer belt due to the switching process might result in image noise. In particular, when the device is switched from the monochrome mode to the color mode, one or more photoreceptor drums are brought into contact with the intermediate transfer belt, which produces a vibration leading to an inappropriate overlapping of color toners on the belt or an irregular pitch between adjacent line images on the belt. However, if the next image forming operation is started after the vibration of the intermediate transfer belt is completely stopped, productivity is greatly reduced.
To minimize such productivity reduction, US Patent Application Publication No. 2003/0038955 A1 discloses an image forming device in which a color image forming operation is started with a delay of a predetermined period of time after the device has been switched from a state where the photoreceptor drums that do not serve to form a monochrome image are separated from the intermediate transfer belt to a state where all the photoreceptor drums are in contact with the belt.
However, such image forming device inevitably reduces productivity where a print job is executed in which monochrome document(s) and color document(s) are alternately printed in the order of printing.